The present invention relates to optical recording elements, including recordable optical elements.
There are many types of known optical recording elements. In many of the elements, the mode of operation requires a recording layer having a high absorption and the recorded marks, often referred to as pits, have low optical density or high reflectivity. The high reflectivity pits are made by ablating away the high absorption recording material, usually exposing an underlying reflective layer.
One of the currently popular forms of optical storage of information is the compact disk or CD. Digital information is stored in the form of high optical density marks or pits on an otherwise reflective background, as read with a focused laser diode operating in the 780-835 nm spectral region. This is the exact opposite of the above described optical recording materials. In this format, the optical information is most often in the form of read only memory or ROM. Optical information is not usually recorded in real time but rather is produced by press molding. In a typical process, the optical recording substrate is first press molded with a master containing the digital information to be reproduced. The thus formed information is then overcoated with a reflective layer and then with an optional protective layer. In those areas having the deformations or pits, the optical density is higher than in those areas not having the deformations.
It is desirable to produce optical recording elements which, when recorded in real time, produce a record that mimics the conventional CD on read out by generating dark marks on a reflective background.
One recently disclosed system of this type is the so called xe2x80x9cPhoto CDxe2x80x9d. In this system, conventional photographic film is first processed in a conventional manner. Then, the images from the film are scanned and digitized. The digitized information is recorded in a CD readable form on an optical recording element. Images can then be played back on a CD type player into a conventional television.
Commercially useful materials of the type described in these references have stringent requirements. The recording layer must be able to couple with incident write laser irradiation to provide features having sufficient contrast. At the same time the unmarked portion of the disk must have sufficient reflectivity to conform to the 70% CD-ROM standard (ISO/IEC 10149). The layer must also have good stability towards light, heat and humidity for acceptable shelf life. Photo CD, for example, is a consumer product and it must be capable of withstanding extreme environments. Between the time the original images are recorded on the Photo CD and the time subsequent images are recorded, the CD might be placed in strong sunlight.
Thus, there is a continuing need for optical recording materials that have the necessary optical characteristics so that they are CD compatible, can couple with incident laser irradiation to form features with sufficient contrast, meet the minimum reflectivity standard, and yet are light stable. It is to a solution to this problem that the present invention is directed.
U.S. Pat. No. 5,426,015 discloses optical recording elements having recording layers containing tetra dyes. These dyes comprise metallized azodianions with cationic dye counterions. These elements exhibit sensitivity that could be improved.
Both FIGS. 1 and 2 illustrate the greater sensitivity of the recording elements of the invention compared to the prior art.
The present invention provides an optical recording element having a transparent substrate and on the surface of said substrate, a recording layer and a light reflecting layer wherein (a) the unrecorded layer is such that the real part of the refractive index (n) at 780 nm, is not less than 1.8, and the imaginary part (k) is not greater than 0.15 and (b) the recording layer comprises one or more tetra dyes having a metallized azo dianion and cationic dye counterions and (c) a recording layer thickness from 225 to 300 nanometers.
The tetra dyes, including mixtures of such dyes, together with thicker recording layers provide improved laser recording sensitivity and superior recording layer performance. Sensitivity is measured by the carrier-to-noise ratio (CNR) at a given power or by the power required to attain a given CNR value.
We have found that recording layer thicknesses prescribed by this invention provide improved sensitivity. The recording process in CD writable element is very complex, we believe that the dye chromaphore is destroyed during the extreme temperatures reached upon laser exposure. We also have microscopic evidence that gases are released and bubbles may form, particularly in thicker dye layers. However, it is not obvious why thick films should show enhanced sensitivity. Indeed, the absorption of thick films is greater, and more light is coupled into the layer, but the amount of material to be altered during recording is increased proportionately. One possible explanation for the enhanced sensitivity is that a greater portion of the reactive layer is removed from the thermally conductive gold layer, allowing for more effective use of the available thermal energy in dye bleaching or gas formation.
The tetra dyes used in the invention form the light absorbing layer in mixtures with other dyes and/or other addenda. Useful tetra dyes have the general structure I: 
R represents hydrogen, alkyl having 1 to 20 carbons, aryl having 6 to 10 carbon atoms;
R1 represents hydrogen or alkyl having 1 to 6 carbon atoms;
R2 represents a hydrogen, alkyl having 1 to 5 carbon atoms, SO2R7 or SO2NHR7 where R7 is alkyl having 1 to 10 carbon atoms, aryl having 6 to 10 carbon atoms hetaryl having 5 to 10 carbon atoms or halogen;
R3 represents hydrogen, alkyl having 1 to 4 carbon atoms or halogen.
R4 represents hydrogen, halogen, alkyl having 1 to 10 carbons, SO2R7 or SO2NHR7;
R5 represents an electron withdrawing group such as CN, SO2R8, SO2NR9R10 where R8, R9 or R10 each independently represents hydrogen, alkyl group from 1 to 5 carbons; aryl having 6 to 10 carbon atoms; hetaryl, having 5 to 10 carbons, cycloalkyl having 5 to 7 carbons;
R6 represents hydrogen; alkyl having 1 to 5 carbons or halogen;,
X represents oxygen, carboxyl or sulfo; and
Dye+ can be any dye that has one or more positive charges.
The tetra dyes are prepared with known procedures and materials described in U.S. Pat. No. 5,426,015.
The following tetra dyes are representative of the dyes used in this invention. The dianionic structures can be selected from Table 1 and the cationic dye structures can be selected from Table 2.
Exemplary tetra dyes formed from the components of Table 1 and Table 2 are presented in Table 3:
The optical elements of the invention comprise a light transmitting, typically pregrooved substrate, a light absorptive layer comprising a dye of the invention overlaying the substrate, a light reflective layer overlaying the light absorptive layer and a protective layer overlaying the light reflective layer. The recording process will produce marks of lower reflectivity than the unmarked areas of the disk when written and read with a diode laser emitting between 770 and 800 nm. It is preferred that the substituents on the dye molecule be selected so that the real part of the complex refractive index (n) of the unwritten light absorptive layer measured with a 780 nm light source is not less than 1.8 and the imaginary part (k) is not greater than 0.15.
The substrate may be any transparent material that satisfies the mechanical and optical requirements. The substrates are generally pregrooved with groove depths from 20 to 250 nm, groove widths 0.2 to 1 xcexcm and a pitch 0.5 to 2 xcexcm . The preferred material is polycarbonate, other materials are glass, polymethylmethacrylate and other suitable polymeric materials.
The preparation of the optical recording element of the invention is achieved by spin coating a tetra dye or a tetra dye mixture, or such compositions with additional dyes, or with addenda from a suitable solvent onto a transparent substrate. For coating, the dye mixture with or without addenda is dissolved in a suitable solvent so that the dye is 20 or less parts by weight to 100 parts of solvent by volume. The dye recording layer of the element is then overcoated with a metal reflective layer under reduced pressure by resistive heating or a sputter method and finally overcoated with a protective resin.
Coating solvents for the dye recording layer are selected to minimize their effect on the substrate. Useful solvents include as alcohols, ethers, hydrocarbons, hydrocarbon halogens, cellosolves, ketones. Examples of solvents are methanol, ethanol, propanol, pentanol, 2,2,3,3-tetrafluoropropanol, tetrachloroethane, dichloromethane, methyl cellosolve, ethyl cellosolve, 1-methoxy-2-propanol, methyl ethyl ketone, 4-hydroxy-4-methyl-2-pentanone. Preferred solvents are alcohols since they have the least effect on the preferred polycarbonate substrates. Mixtures of solvents can also be used.
Useful addenda for the recording layer include stabilizers, surfactants, binders and diluents.
The reflective layer can be any of the metals conventionally used for optical recording materials. Useful metals can be vacuum evaporated or sputtered and include gold, silver, aluminum and copper and alloys thereof.
The protective layer over the reflective layer is similarly conventional for this art. Useful materials include UV curable acrylates.
One preferred protective layer is disclosed in U.S. Pat. No. 5,312,663 in the names of Kosinski and Amell. This patent discloses a two layer structure in which the layer adjacent to the reflective layer is spin coated and the a second layer is screen printed.
An intermediate layer, to protect the metal layer from oxidation, can also be present.
The element of the invention can have prerecorded ROM areas as described in U.S. Pat. No. 4,940,618. The surface of the substrate can have a separate heat deformable layer as described in U.S. Pat. No. 4,990,388. Other patents relating to recordable CD type elements are U.S. Pat. Nos. 5,009,818; 5,080,946; 5,090,009; 4,577,291; 5,075,147; and 5,079,135.